A disk drive is a digital data storage device that stores information on concentric tracks on a storage disk. The storage disk is coated on one or both of its primary surfaces with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. During operation of a disk drive, the disk is rotated about a central axis at a constant rate. To read data from or write data to the disk, a magnetic transducer (or head) is positioned above (or below) a desired track of the disk while the disk is spinning.
Writing is performed by delivering a polarity-switching write current signal to the magnetic transducer while the transducer is positioned above (or below) the desired track. The write signal creates a variable magnetic field at a gap portion of the magnetic transducer that induces magnetically polarized transitions on the desired track. The magnetically polarized transitions are representative of the data being stored.
Reading is performed by sensing the magnetically polarized transitions on a track with the magnetic transducer. As the disk spins below (or above) the transducer, the magnetically polarized transitions on the track induce a varying magnetic field into the transducer. The transducer converts the varying magnetic field into a read signal that is delivered to a preamplifier and then to a read channel for appropriate processing. The read channel converts the read signal into a digital signal that is processed and then provided by a controller to a host computer system.
When data is to be written to or read from the disk, the transducer must be moved radially relative to the disk. In a seek mode, the transducer is moved radially inwardly or outwardly to arrange the transducer above a desired track. In an on-track mode, the transducer reads data from or writes data to the desired track. The tracks are typically not completely circular. Accordingly, in the on-track mode the transducer must be moved radially inwardly and outwardly to ensure that the transducer is in a proper position relative to the desired track. The movement of the transducer in on-track mode is referred to as track following.
Modern hard disk drives employ a dual-actuator system for moving the transducer radially relative to the disk. A first stage of a dual-actuator system is optimized for moving the transducer relatively large distances. A second stage of a dual-actuator system is optimized for moving the transducer relatively small distances. The present invention relates to hard disk drives having dual-stage actuator systems.
FIGS. 1 and 2 depict a mechanical portion of a disk drive 10. The disk drive 10 further comprises control electronics typically including a preamplifier, a read/write channel, a servo control unit, a random access memory (RAM), and read only memory (ROM), spindle motor, and dual-stage driving electronics. The electronic portion is or may be conventional and will not be described herein beyond what is necessary for a complete understanding of the present invention.
FIGS. 1 and 2 show that the mechanical portion of the disk drive 10 includes a disk 12 that is rotated by a spin motor 14. The spin motor 14 is mounted to a base plate 16. The disk drive 10 includes at least one and typically a plurality of disks 12, each with one or two recording surfaces. During use, the disk 12 is rotated about a spindle axis A.
The disk drive 10 further comprises what is commonly referred to as a head 18. The head 18 comprises or supports the magnetic read/write transducer described above; the head 18 will be referred to herein as the component of the disk drive 10 that reads data from and writes data to the disk 12.
FIGS. 1 and 2 further illustrate a positioning system 20 of the disk drive 10. The positioning system 20 comprises a bearing assembly 22 that supports at least one actuator arm assembly 24. The actuator arm assembly 24 supports the head 18 adjacent to one recording surface 26 of one of the disks 12. Typically, the bearing assembly 22 will support one actuator arm assembly 24 and associated head 18 adjacent to each of the recording surfaces 26 of each of the disks 12. The actuator arm assemblies 24 allow each head 18 to be moved as necessary to seek to a desired track 28 in seek mode and then follow the desired track 28 in track following mode.
The positioning system 20 depicted in FIGS. 1 and 2 is a dual-stage system. Accordingly, each actuator arm assembly 24 comprises a first actuator structure 30 and a second actuator structure 32. For ease of illustration, FIGS. 1 and 2 depict the first and second actuator structures 30 and 32 as comprising first and second elongate actuator arms 34 and 36, respectively, and the actuator structures 30 and 32 may be implemented as shown in FIGS. 1 and 2.
The actuator structures 30 and 32 may, however, be implemented using other structures or combinations of structures. For example, the first actuator structure 30 may comprise an elongate arm that rotates about a first axis B, while the second actuator structure 32 may comprise a suspension assembly rigidly connected to a distal end of the first actuator. In this case, the first actuator is able to rotate about an actuator axis, while the head 18 would be suspended from the second actuator for linear movement along the disk radius relative to the position of the first actuator. The actuator structures 30 and 32 may thus take any number of physical forms, and the scope of the present invention should not be limited to the exemplary actuator structures 30 and 32 depicted in FIGS. 1 and 2.
Conventionally, the bearing assembly 22 is also considered part of the first actuator structure 30. In particular, the bearing assembly 22 supports a proximal end 40 of the first actuator arm 34 for rotation about a first axis B, while a distal end 42 of the first actuator arm 34 supports a proximal end 44 of the second actuator arm 36 for rotation about a second axis C. In this case, the head 18 is supported on a distal end 46 of the second actuator arm 36.
FIG. 2 also illustrates that the exemplary actuator structures 30 and 32 of the positioning system 20 form part of a first actuator 50 and a second actuator 52. For the purposes of the following discussion, the first actuator 50 is identified as a voice coil motor (VCM) and the second actuator 52 is identified as a piezoelectric transducer (PZT). However, the actuators 50 and 52 may be formed by any device capable of moving based on an electrical control signal as will be described below.
In particular, based on a first actuator control signal, the first actuator 50 moves the first actuator arm 34 to change an angular position of the head 18 relative to the first axis B. The second actuator 52 is supported by the distal end 42 of the first actuator structure 30 to rotate the head 18 about the second axis C based on a second actuator control signal. In FIG. 2, an angular position of the first actuator arm 34 is represented by reference character D, while an angular position of the second actuator arm 36 is represented by reference character E.
A range of movement “S” associated with the second actuator structure 32 is defined by the stroke “s+” and “s−” in either direction relative to a neutral position D defined by the first actuator arm 34. The term “actual displacement” (ds in FIG. 2) refers to the angular difference at any point in time of the head 18 relative to the neutral position as defined by the position D of the first actuator structure 30. When the head 18 is in the neutral position, the actual displacement of the second actuator arm 36 is zero.
FIG. 2 further identifies arbitrary first and second tracks 28a and 28b on the disk 12. The actuator arm assembly 24 is shown in an initial position by solid lines and in a target position by broken lines; the first track 28a will thus be referred to as the “initial track” and the second track 28b will be referred to as the “target track”. It should be understood that the terms “initial track” and “target track” are relative to the position of the head 18 before and after a seek operation. Any track 28 on the disk 12 may be considered the initial track or the target track depending upon the state of the disk drive 10 before and after a particular seek operation.
FIG. 3 contains a block diagram of a servo system 60 incorporating a conventional two-stage actuator system. The servo system 60 will typically be embodied as a software program running on a digital signal processor, but one of ordinary skill in the art will recognize that control systems such as the servo system 60 described herein could be implemented in hardware.
The servo system 60 comprises a first stage 62 and a second stage 64. As described above, the disk 12 defines a plurality of tracks 28 in the form of generally concentric circles centered about a spindle axis C. The first stage 62 controls the VCM 50 and the second stage 64 controls the PZT 52 to support the head 18 adjacent to a desired one of the tracks 28. The first and second actuator control signals are generated as part of this larger servo system 60.
More specifically, an input signal “R” is combined with a position error signal “PES” by a first summer 70. The second stage position signal Y2 is indicative of an actual position of the actuator 52 of the second stage 64, and a second stage position estimate signal “Y2est” is indicative of an estimated position of the actuator 52 of the second stage 64. The second summer 72 combines the second stage position estimate signal “Y2est” and the output of the first summer 70. A first stage position signal “Y1” is indicative of the actual position of the first actuator 50 of the first stage 62. A third summer 74 combines the first and second stage position signals “Y1” and “Y2”. System disturbances “d” are represented as an input to the third summer 74. The position error signal “PES” thus represents the combination of the first and second position signals “Y1” and “Y2” with any system disturbances “d”.
The source of the input signal “R” and the first and second stage position signals “Yi” and “Y2” is or may be conventional and will be described herein only to the extent necessary for a complete understanding of the present invention. Briefly, each of the tracks 66 contains data sectors containing stored data and servo sectors containing servo data. The servo data identifies each individual track 66 to assist in seek operations and is also configured to allow adjustment of the radial position of the head 18 during track following. As is conventional, a servo demodulation unit generates the position error signal “PES” and the first and second stage position signals “Y1” and “Y2” based on the servo data read from the disk 12. The input signal “R” is generated by a host computer or is simply zero during track following.
Referring now back to the servo system 60, the frequency response of the first stage 62 may be different from that of the second stage 64. For example, the process of manufacturing a second actuator 52 using PZT technology typically results in the second stage 64 having a wider gain distribution than the first stage 62 using VCM technology. The overall bandwidth of the system 60 is thus determined by the wider bandwidth associated with the second stage 64. The gain variation of the second stage 64 thus directly affects the bandwidth and stability margins of the entire system 60. The need thus exists to calibrate the gain of the second stage 64 to improve system bandwidth and stability margins.
In addition, the second stage 64 may drive the exemplary PZT actuator 52 using a voltage driver instead of a charge driver. An actuator using a voltage driver is susceptible to gain drifts with changing temperatures. The need thus further exists for a gain calibration system for the second stage 64 to compensate for such temperature related gain variations.
Gain calibration of the second stage 64 generally requires knowledge of how a particular second transducer responds to a known input signal. The Applicants are aware of several calibration methods that have been proposed for calibrating the gain of the second stage 64 of the hard drive dual stage actuator.
The first calibration method is to perform complete open loop acceleration/deceleration of the second stage actuator structure 32. This method is similar to the process used to calibrate the first stage actuator structure 30. The stroke (“s+”) of the second stage actuator structure 32 is, however, relatively limited, typically on the order of approximately 1 μm. With disturbances “d” commonly on the order of tens of tracks, the motion of the second stage actuator structure 32 can be buried in the noise. This technique thus does not work well when applied to the second stage actuator structure 32 of a hard disk drive.
The second calibration method is to introduce a single frequency point DFT into the second stage 64. This second approach can be implemented with a single stage servo (the first stage loop closed) or with a dual-stage servo (both loops closed). The block diagram of the servo system 60 is modified as shown in FIG. 4 when this calibration method is used. As shown in FIG. 4, the first stage 62 comprises a first stage controller 80 and the first stage actuator 50. The second stage 64 comprises a second stage controller 82, an estimator circuit 84, a notch filter 86, and the second stage actuator 52. The first and second stage controllers 80 and 82 are thus conventionally considered part of the servo system 60. During calibration an excitation signal μAin is input to the notch filter 86 instead of the output of the second stage controller 82.
Because the motion of the second stage actuator structure 32 cannot be directly measured, the second stage response is calculated according to one of the following formulas (1) or (2). In particular, if the second stage controller 82 is inactive, the second stage response is calculated according to the following formula (1):                               PES          ⁡                      (                          j              ⁢                                                           ⁢              ω                        )                                    μ          ⁢                                           ⁢                                                    A                in                            ⁡                              (                                  j                  ⁢                                                                           ⁢                  ω                                )                                      ·                                          ETF                VCM                            ⁡                              (                                  j                  ⁢                                                                           ⁢                  ω                                )                                                                        (        1        )            
If the second stage controller 82 is active, the second stage response is calculated according to the following formula (2):                               PES          ⁡                      (                          j              ⁢                                                           ⁢              ω                        )                                    μ          ⁢                                           ⁢                                                    A                in                            ⁡                              (                                  j                  ⁢                                                                           ⁢                  ω                                )                                      ·                                          ETF                                  Dual                  -                  stage                                            ⁡                              (                                  j                  ⁢                                                                           ⁢                  ω                                )                                                                        (        2        )            
During application of these formulas (1) and (2) to calibrate the DC gain, ω is typically chosen from the range of between approximately 100 Hz and 500 Hz. Of these two formulas, formula (2) is less frequently used than formula (1) because, if the gain is way off the nominal value, the dual-stage servo may not be able to stay on track.
Both of these formulas (1) or (2) use the error transfer function (ETF) of the servo loop as the devisor and thus require measurement of the error transfer function. The process of measuring the error transfer function complicates implementation of the servo system 60 and increases the likelihood that the calibration will be inaccurate.
Even if the nominal VCM plant is used and it is assumed that, at low frequencies, the gain variation of the VCM is negligible, this second calibration approach is still susceptible to the problem of low coherence. As shown in formulas (1) and (2), the second stage 64 is filtered by the error transfer function, which has high attenuation at low frequencies. The maximum stroke (“s+”) of the second stage actuator structure 32 is on the order of approximately 1 μm, and is even lower if a low voltage driver is used for cost reduction purposes. Accordingly, when the excitation signal eventually shows up in the “PES”, the excitation signal has been substantially attenuated. In practice, attenuation by the error transfer function adds approximately 1 dB of measurement inconsistency beyond the error introduced by the process of measuring the error transfer function.
A need thus exists for improved positioning systems and methods for a dual-stage actuator of a disk drive and, in particular, for improved calibration systems and methods for the second stage of such positioning systems and methods.